The US did not get involved because of the sinking of the Lusitania, that occurred in 1915, which we did not enter the war until 1917. In fact the Lusitania was carrying arms to England. The United States (Wilson did so with out the citizens knowledge) were indeed shipping arms and goods to England during World War One there is no denning that fact. The Lusitania was carrying weapons and explosions, that is why the ship sunk so fast.
When the US started to supply the English, even though they were considered neutral, is an act of war. From the beginning, Wilson wanted to make sure the US would be on the British side, if they are forced in to the war. Prior to the war the US traded more goods with the German empire than the British empire. However, when the War started the British Blockaded the German shipping. Which forced the US to give up the Freedom of the Seas. This is considered an act of war, because the US believed that the Seas are free for any nation to use. When the english permitted the trade with Germany the US ignored the situation. This was the first step for US picking a side. Almightily, dragging them in the war.
After the sinking of the Lusitania, the US and Germany came to agreement that Germany will not sink any civilian ships, unless there are carrying weapons. Part of agreement, was that the Germans had to have their U-Boats surface and board the ships to check for weapons. If the ship had arms, all citizens must aboard escape boats and then the Germans could sink the ship. However, the English were desperate for US entering the War. So the British began to fly US Ships (which is piracy and declaration of war for flying other national flags), on the warship that they disguised as citizen ships. When the U-Boats surfaced the British would sink the U-boats. Therefore, the Germans went back to sinking citizen ships without warning if believed they were carrying arms. The US once again ignored the British acts of war.
Also the American did decode nor the ones that found the Zimmerman Code, it was actually given to the US by the British. The British decoded the German telegrams and reveled the Zimmerman Code to the US. Which the Mexican Government declined right away, because they were in a civil war and would not be able to fight the US also. Wilson knew all this information, but still declared war on Germany. Germany sent the Code for a final desperate attempt to divert the US from assisting England. Germany was starving at this point from the Blockade and need to stop the US from assisting the Allies. They viewed that US was already in the war and on the Allies side, so they felt like they had nothing to lose.
The Zimmerman Code, Propaganda, close relations with the British and Democracy was the major reasons for entering the war with the Allies.
Zimmerman Code was a threat and an insult to the US.
Propaganda of the U-Boat sinking citizen ships and killing civilians. (US citizen did not know the real reasons).
Wilson is British decent and looking at the English as more likely future Allies.
Democracy: the US had to support a fellow Democracy nation if they entered the war. The US could not support a monarchy, because it would go against everything they believe and stand for.